Rescreeining selected parts of a halftone image

ABSTRACT

A method for enhancing appearance of a halftone image ( 204 ) for imaging on a flexographic plate ( 108 ) includes retrieving the halftone image from a data storage device ( 132 ); setting a minimal dot size value for printing; analyzing the halftone image with a computer ( 130 ); detecting areas ( 404 ) in the halftone image populated with a plurality of dots smaller than the minimal dot size value; replacing the plurality of dots with a reduced set of dots wherein each of the reduced set of dots are larger in size ( 504 ) than the plurality of dots and wherein each of the reduced set of dots maintains an original geometric characteristics of the plurality of dots; and saving the reduced set of dots.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned copending U.S. patent applicationSer. No. ______ (Attorney Docket No. K001080US01NAB), filed herewith,entitled RESCREENING SELECTED PARTS OF A HALFTONE IMAGE, by Krol; thedisclosure of which is incorporated herein.

FIELD OF THE INVENTION

The present invention relates to the field of graphic arts, printing andpublishing technologies, specifically, to image reproduction systemscharacterized by limited numbers of levels of optical parameters.

BACKGROUND OF THE INVENTION

In graphic arts technology there is a need for reproduction ofcontinuous tone images by an imaging device. The imaging device, forexample, a laser based imager is capable of producing a limited numberof levels representing optical parameter (in most common cases only twolevels are used—black and white). This goal is achieved by applying aprocess called screening, wherein a gray tone value which represents apixel to be screened in the original image is simulated by means ofvarying relative area covered by dark elements (pixels) as compared tolight elements.

Different screening methods exist. In one of the most common, an area ofan image is reproduced by subdividing it into equal, periodicallyrepeated sub-areas called mesh cells, containing variable-size darkelements (printing dots, alternatively called halftone dots). Relativearea coverage is defined as ratio of dark element area to a mesh cellarea. Such screening method is commonly called Amplitude Modulation (AM)screening. Such screen, with a regular, usually square grid structure,is characterized by a screen period and a screen angle. The reciprocalof this period is called screen frequency or screen ruling.

Particular problems arise when reproducing characteristics of printingdots which are size dependant. Examples of such processes areflexographic, offset and xerographic printing processes.

In flexographic printing, the size of the smallest halftone printing doton a printing plate that can be consistently reproduced on press isusually around 40 microns in size. Below this size, halftone dots tendto print unevenly, and may also include drastic increase in size andproduce large blot artifacts, or not printing at all. For commonly usedline ruling of 120 lpi and commonly used device resolutions of 2400 or2540 dpi, 40 micron halftone dot corresponds to area coverage of 3%; forline ruling 1500 lpi, it corresponds to area coverage around 4%. Thismay result in discontinuity, with annoying artifacts in certain casesespecially in the highlight parts of printed images. The high dot gainthat is associated with flexographic printing process enhances theeffect and exacerbates the problem.

In the offset printing process, the minimal halftone printing dot thatcan be reproduced consistently can often be as small as 10 micron. Forlower quality paper and high-speed printing presses, for example, innewspaper printing, the same fundamental problem discussed above exists.A similar situation exists in electro-photographic printing, whereinminimal printing dot size is often defined by physical characteristicsof toner particles.

One solution for the above problems is using Frequency Modulation (FM)screening techniques with controlled minimal dot size, so-called “greennoise” frequency modulation, shown in U.S. Pat. No. 5,689,623 (Pinard),or with controlled midtone clustering, so-called “second order”frequency modulation, shown in U.S. Pat. No. 5,579,457 (Hall). Whilesolving the problem of highlight region reproduction, frequencymodulation introduces its own drawbacks. Relatively rough feature sizeneeded for proper highlights reproduction, often leads to grainyappearance both in highlights and in the midtones areas. Additionally,high circumference-to-area ratio inherent for FM generated printing dotsleads to significantly higher dot gain compared to AM halftonescreening. Considering that flexographic printing process is alreadycharacterized by high dot gain, FM screening may lead to significantcontraction of the dynamic range for printed images.

Another solution is known as the “double dot” technique or “Respiscreen.” According to this technique, the extremes of tone scale,highlights and/or shadows, are rendered with halftone dots that are laidon a grid with the same angle, but with the frequency of the square rootof the halftones in the rest of tone scale, thus halving the number ofhalftone dot and, consequently, doubling the size of each dot. Thisrenders the transition area between extreme and main part of tone scalewith halftone dots of two different sizes placed in checkerboardpattern. While moving the cutoff value for non reproducible part of theimage farther to extreme parts of tone scale, it still does notcompletely solve the problem. Moreover, by introducing additional screenfrequencies, such technique may produce highly undesirable moiré effectsin multi-colored images in case of regions where part of colorseparations are in extreme parts of the tone scale and other separationsare in non-extreme part of the color scale.

In order to overcome the deficiencies above stated, an approach wasproposed by U.S. Pat. No. 5,766,807 (Delabastita et al.). This method isknown as “hybrid screening.” In this approach, a “supercell” thresholdmatrix suitable for periodically tiling a plane is defined in such a waythat it contains a plurality of locations for halftone dot centers andis filled with threshold values. When the matrix is used for screening acontone image, in the extreme parts of the tone scale halftone dot ofpredefined minimal size are produced, whereas in the remaining part ofhalftone dot centers no halftone dot are produced at all. This isperformed in such a way that the area coverage for a whole supercellarea corresponds to a tone value in the contone image. In other words,instead of modulating halftone dot size, below predefined dotpercentage, dot size is kept constant but dot number is modulated as afunction of tone value; accuracy of tone representation being defined bypredefined minimal halftone dot size and count of halftone dot centersin supercell threshold matrix.

While free of most undesirable effects of previous solutions, thisapproach still has some problems. Notably, the supercell-based patternis prone to grainy and “noisy” appearance; relatively rough quantizationsteps limited by number of halftone dot centers in a supercell thresholdmatrix may produce banding effects in vignette parts of image;“orphaned” and incomplete halftone dots still may produce undesirable“blot-like” artifacts; and supercell-based approach limits availablenumber of screen angle/screen frequency combinations to those withrational tangent angles.

SUMMARY OF THE INVENTION

Briefly, according to one aspect of the present invention a method forenhancing appearance of a halftone image for imaging on a flexographicplate includes retrieving the halftone image from a data storage device;setting a minimal dot size value for printing; analyzing the halftoneimage with a computer; detecting areas in the halftone image populatedwith a plurality of dots smaller than the minimal dot size value;replacing the plurality of dots with a reduced set of dots wherein eachof the reduced set of dots are larger in size than the plurality of dotsand wherein each of the reduced set of dots maintains an originalgeometric characteristics of the plurality of dots; and saving thereduced set of dots to a storage device.

The invention and its objects and advantages will become more apparentin the detailed description of the preferred embodiment presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents in diagrammatic form a prior art imaging systemadapted to expose flexographic plates;

FIG. 2 represents in diagrammatic form a prior art halftone image readyto be exposed;

FIG. 3 represents in diagrammatic form an enhanced halftone image,enhancing the image shown in FIG. 2;

FIG. 4 represents in diagrammatic form a small dots area from a halftoneimage to be enhanced; and

FIG. 5 show in diagrammatic form enhanced area of small dots shown inFIG. 4, the small dots were replaced with a set of fewer dots whereineach new dot is larger in size.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, specific details are set forth inorder to provide a thorough understanding of the disclosure. However, itwill be understood by those skilled in the art that the teachings of thepresent disclosure may be practiced without these specific details. Inother instances, well-known methods, procedures, components and circuitshave not been described in detail so as not to obscure the teachings ofthe present disclosure.

While the present invention is described in connection with one of theembodiments, it will be understood that it is not intended to limit theinvention to this embodiment. On the contrary, it is intended to coverall alternatives, modifications and equivalents as covered by theappended claims.

In order to improve the reproduction characteristics of a halftonedimage by of controlling halftone dot size and spatial distribution inextreme tone scale parts of said image, a halftone image alreadyscreened by means of traditional Amplitude Modulation (AM) screening isconceptually represented as rectangular array of black and white pixels.The present invention is not limited by any specific type, algorithm ormethod of AM screening and utilizes only two basic characteristics of AMscreening, namely parameters of a periodic grid, its angle and itsfrequency represented by its line ruling, i.e. mesh, and derivedparameter such as res/mesh ratio, equal to side length of single meshcell in units of single pixel size.

In order to process the screened image 204 shown in FIG. 2. The screenedimage 204 is fetched from storage element 132 (FIG. 1) by computer 130.A “natural” system of coordinates is defined, where X axis runs alongimage lines and Y axis is perpendicular to Y axis, and unit of measureis single pixel size, and its “shifted” system of coordinates, where X′axis and Y′ axis run along mesh grid directions that is, rotated byscreen angle related to natural system of coordinates and unit ofmeasure stated res/mesh ratio.

Additionally, DS1 is defined as minimal reliably reproducible halftonedot size (in pixels) and DS2>=DS1 as halftone dot size (in pixels)corresponding to cutoff area coverage defining transfer from normal to“extreme” parts of tone scale. Separate halftone dots contained in thehalftone image are identified and are processed in following the manner:if size of halftone dot is >=DS2, this halftone dot is not modified.

If size of halftone dot DS is <DS2, coordinates of its center of gravityXc and Yc are calculated in natural system of coordinates, thesecoordinates are transformed into X′c and Y′c in shifted coordinates, andfrom these coordinates corresponding mesh cell indices Mx=floor(X′c) andMy=floor(Y′c) are obtained. To these indices operator F(DS, mx, my) isapplied, which returns 1 or 0 in such a way that average value(1/Nds)ΣF(DS, mx, my)*DS2, where sum is done for all halftone dots ofsize DS, is substantially equal to DS. Operator F can be analyticalexpression or simple lookup operation into predefined array. If operatorF returns 0, we remove halftone dot by replacing all its pixels withpixels of an opposite color. If operator F returns 1, halftone dots arereplaced with the halftone dots of the same shape and the same center ofgravity, but of size DSn, where DSn is a value chosen out of array ofvalues [DSn1, DSn2, . . . , DSnN] such as DSn1>=DS1,DSnX<DSn(X+1) andaverage value (1/N)Σ(DSnX)==DS2.

This method ensures both consistent reproduction of halftone dots inregions with extreme values of tone scale via printing process andsmooth, non-grainy, artifact-free appearance of said regions andtransition regions from extreme values of tone scale to midtone ones.

In a preferred embodiment of the invention, an array of values [DSn1,DSn2, . . . , DSnN] contains four elements—DSn1, DSn2, DSN3 and DSn4such that the size of element DSn1 is equal to DS1, minimal reliablyreproducible halftone dot size (in pixels); size of elements DSn2, DSn3and DSn4 is DS1+1, DS1+2 and DS1+3 pixels, respectively. Cutoff size DS2in a preferred embodiment is defined as DS1+2 pixels.

Operator F(DS, mx, my) in the preferred embodiment is defined as acompare operation of a value derived from halftone dot size DS with avalue from square lookup array B[N][N] where array size N is 2̂n, n>>8.Lookup array B[N][N] is uniformly filled with integer values from 0 upto M=2̂m−1, m>=10 in such a way that a) array B[N][N] exhibits wraparoundproperties in both horizontal and vertical directions and b) when usedas threshold array, results of threshold operation at any given levelfrom 0 up to M exhibits blue-noise characteristics.

Screen angle is denoted as α, screen line ruling as mesh, imageresolution as res and screen cell side length as r2 m=res/mesh. Givenabove these definitions, the preferred embodiment of the invention canbe represented in pseudo code in following way:

  While scanning a halftoned 1-bit image;      identify halftone dots;     assign each pixel to its respective dot.      For each halftone dot      If ( dot size in pixels DS >= DS2)         continue to nexthalftone dot without modification;   else modify dot as described below:  {   // calculate center of gravity   Xc = (1/DS)ΣXpix (summation forall pixels of halftone dot)   Yc = (1/DS)ΣYpix (summation for all pixelsof halftone dot)   // transform coordinates into shifted system ofcoordinates   X′c = Yc*sin(α)+Xc*cos(α);   Y′c = Yc* cos(α)−Xc*sin(α);  // Normalize to shifted coordinates units   X′c = X′c/r2m;   Y′c =Y′c/r2m;   // Calculate mesh cell indices   Mx = floor(X′c);   My =floor(Y′c);   // Calculate offset into lookup array   Nx = Mx%N;   Ny =My%N;   // Calculate effective area coverage for current halftone dot  Coverage = DS*100/(r2m*r2m);   // Calculate compare value forthresholding operation with value //   from lookup array B  cutoffCoverage = DS2*100/( r2m*r2m);   percentToKeep = 100.* Coverage/ cutoffCoverage;   arrayCompareVal = (100.-percentToKeep)*(M+1) /100;if (B[Nx][Ny]> arrayCompareVal)      then remove halftone dot;      else     {      Stochastically choose dot size from array of dot size values     Put halftone dot of chosen size with the same center of gravity  as original halftone dot      } }

FIG. 3 shows the halftone image after enhancement. Numeral 304 shows anarea of dots 304 which replaced the area of dots 204. The replaced dots304 include a smaller number of dots compared to dots 204 whichoriginally populated the area in FIG. 2. Each of the newly created dots304 are larger in size than the original dots 204 and maintain thegeometric characteristics of the dots 304 in terms such as center ofgravity and angle.

FIG. 4 shows a zoomed in area of small dots 404, wherein FIG. 5 showsdots 504 which are replacing dots 404. It can seen much more clearlythat the number of the replaced dots are fewer and each of the new dots504 is larger than the original dots 404, however the geometrical layoutof the dots 504 is maintained to be similar to the those shown for dots404.

FIG. 1 shows an imaging system 100. The imaging system 100 includes animaging carriage 112 mounted on an imaging head 120. The imaging head120 is configured to image on a flexographic plate 108 mounted on arotating cylinder 104. The carriage 112 is adapted to move substantiallyin parallel to cylinder 104 guided by an advancement screw 116.

The enhanced halftone 304 is delivered by controller 128 from computer130 to imaging head 120 of imaging system 100, and is further exposed onflexographic plate 108, by imaging system 100 to form imagedflexographic plate.

While the invention has been described with respect to a limited numberof embodiments, these should not be construed as limitations on thescope of the invention, but rather as exemplifications of some of thepreferred embodiments. Other possible variations, modifications, andapplications are also within the scope of the invention. Accordingly,the scope of the invention should not be limited by what has thus farbeen described, but by the appended claims and their legal equivalents.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention.

PARTS LIST

-   100 imaging system-   104 rotating cylinder-   108 flexographic plate-   112 carriage-   116 screw-   120 imaging head-   128 controller-   130 computer-   132 data storage device-   204 AM screened image before treatment-   304 screened dots after treatment-   404 area of small dots previously screened-   504 area of screened dots after treatment

1. A method for enhancing appearance of a halftone image for imaging ona flexographic plate comprising: retrieving the halftone image from adata storage device; setting a minimal dot size value for printing;analyzing the halftone image with a computer; detecting areas in thehalftone image populated with a plurality of dots smaller than theminimal dot size value; replacing the plurality of dots with a reducedset of dots wherein each of the reduced set of dots are larger in sizethan the plurality of dots and wherein each of the reduced set of dotsmaintains an original geometric characteristics of the plurality ofdots; and saving the reduced set of dots to the storage device.
 2. Themethod according to claim 1 wherein the original geometriccharacteristics is a center of gravity of a dot.
 3. The method accordingto claim 1 wherein the original geometric characteristics is an angle atwhich the dots are positioned.
 4. The method according to claim 1wherein the original geometric characteristics is a mesh of the dot. 5.The method according to claim 1 wherein each of the reduced set of dotsis enlarged according to a dot size wherein the dot size is determinedin stochastic manner.
 6. The method according to claim 1 comprising:printing the halftone image.